It is necessary to control the light system of a trailer by the light signal system of the towing vehicle to communicate the intentions of the driver of the towing vehicle to the drivers of following vehicles. Straightforward attempts to control the trailer system such as by wiring corresponding lamps of the trailer and towing vehicle in parallel have severe disadvantages. For example, parallel wiring increases current requiring the replacement of the existing vehicle turn signal flasher with a heavy duty flasher.
Various adapter units have been proposed for interfacing the brake and turn signal system of a vehicle with a trailer system of the type have a brake and turn signal lamp on each side of the trailer. Some include circuitry making use of solid state components, such as silicon controlled rectifiers, which may be susceptible to electrical surges resulting in their unreliable operation. For further information regarding the structure and operation of such an adapter unit, reference may be made to U.S. Pat. No. 4,006,453.
Another proposed adapter unit avoids the surge problem by employing a pair of special bucking type relays the coils of which are interconnected with the vehicle brake and turn signal system, and the contacts of which are interconnected to the trailer lamp system. As the trailer lamp system is served by a different circuit than that including the turn signal flasher, no heavy duty flasher is required. The coil of each relay is made up of two coil portions wound in bucking relationship. One of the coil portions is connected to the turn signal switch while the other coil portion is connected to the brake switch. The coil portions are commonly connected to ground. When only the brake is depressed, first coil portions of the two relays are energized causing closing of the sets of contacts to energize both trailer lamps. With only the left turn signal on, only a second coil portion of a first relay is energized causing the left trailer lamp to flash simultaneously with the vehicle's left stop lamp. However, when both the brake is depressed and the left turn signal is on with the flasher in the "on" portion of its cycle of operation, both coil portions of the first relay are energized. As they are wound in bucking relationship, the magnetic fields produced substantially cancel and the contacts remain open so that the left trailer light is deenergized. However, when the flasher is in the "off" portion of its cycle of operation, the second coil portion of the first relay is deenergized and the energized first coil portion of the first relay can close the contacts. The result is that when both the left turn signal and brake are actuated, the left trailer lamp flashes 180.degree. out of phase with the vehicle turn signal lamp. For further information regarding the structure and operation of this adapter unit, reference can be made to U.S. Pat. No. 4,064,413.